1. Field of the Invention
This invention relates to wire bonders and a method of wire bonding.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, one of the generally required procedures is that of bonding wire between a pair of wire bonding locations, these wire bond location usually involving bond pads on a semiconductor chip and lead fingers of a lead frame. This procedure is generally accomplished by providing a ball bond onto the bond pad of the semiconductor chip and a stitch bond onto the lead finger of the lead frame, both of which are provided, at least in part, using ultrasonic or thermosonic heating techniques.
With the increasing demand for lower temperature thermosonic bonding processes, the stitch bonds become increasingly critical from the standpoint of reliability. Due to the inherent motion of ultrasonic energy being parallel to the axis of the ultrasonic transducer which provides the ultrasonic energy, wires bonded in a direction normal to the axis of the ultrasonic transducer have relatively high stitch flare area (meaning that the stitch bond spreads out on the surface to which the bond is being made) and have relatively high tensile strength as compared with stitch bonds formed wherein the ultrasonic energy moves in a direction parallel to the wire bonding direction, such bonds displaying relatively low tensile strength and having a relatively smaller stitch area.
To compensate for this known effect, in the prior art, the ultrasonic power bond force and bond time are programmed to vary, depending upon the wire angle or position with respect to the axis of the ultrasonic transducer. However, this approach fails to provide the optimum solution since the direction of the ultrasonic energy still remains the same, namely normal or with a component normal to the direction of the ultrasonic energy.